I. Field of the Invention
The present invention relates to a method and apparatus for forming an oblique groove in semiconductor devices and, more particularly, a method and apparatus for forming a ring-shaped oblique groove in semiconductor rectifier elements and the like for electric power so as to enhance their voltage durable characteristic.
II. Description of the Prior Art
The structure of conventional semiconductor rectifier element for electric power will be described referring to a longitudinally-sectioned view shown in FIG. 1. A semiconductor rectifier element 1 comprises a cathode layer (N.sup.+ -type layer) 4 with which a cathode electrode 3 is resistance-contacted, a cathode neighboring layer (P-type layer) 5 adjacent to the cathode layer 4, a layer (N-type layer) 6 adjacent to the cathode neighboring layer 5 and having a high ratio of resistance, and an anode layer (P-type layer) 8 interposed between the high ratio resistance layer 6 and an anode electrode 7. Therefore, PN junctions J1, J2 and J3 are formed, respectively, at the borders among these four layers. In addition, an insulating groove 2 is formed in the circumference of rectifier element 1. The rectifier element 1 is of truncated conical shape. So, the area of circular cross-sectioned surface, perpendicular to the axis of the rectifier element, of P-type layer 8 is larger than that of N-type layer 6 at the PN junction J3. As is well known, the reason why the cross-sectioned area of P-type layer is made larger than that of N-type layer is to enhance the voltage durable characteristic of the rectifier element. When the cross-sectioned area of P-type layer 5 is similarly made larger than that of N-type layer 6 at the PN junction J2, the voltage durable characteristic of the rectifier element can be further enhanced. In order to achieve this, the ring-shaped insulating groove 2 is sloped with its bottom directed toward the center of the rectifier element 1, as shown in FIG. 1. Therefore, the portion of the element located inside the ring-shaped groove 2 becomes reverse conical, as shown in FIG. 1, and the cross-sectioned area of P-type layer 5 becomes larger than that of N-type layer 6, thus enhancing the voltage durable characteristic of the rectifier element. The cross-sectioned area of N-type layer 6 becomes larger than that of P-type layer 5 at the portion of rectifier element 1 located outside the ring-shaped groove 2. However, this causes no problem because the voltage durable characteristic of the rectifier element 1 is influenced only by that portion of rectifier element inside the ring-shaped groove 2 where the cathode electrode 3 and the cathode neighboring layer 4 are present.
The shape of ring-shaped oblique groove 2 influences the voltage durable characteristic of semiconductor rectifier element. Namely, the voltage durability becomes larger as the width W of the opening of oblique groove 2 becomes larger. Further, the voltage durability also becomes larger as the radius of curvature R of the bottom of groove 2 becomes smaller. Furthermore, the angle .theta. formed by the wall 2a of oblique groove 2 located on the center side of rectifier element 1 and the PN junction J2 also influences the voltage durable characteristic of the rectifier element.
The ring-shaped oblique groove 2 is conventionally formed as follows using the sand blast process. The rectifier element of truncated conical shape is held by a rotary device and rotated concentrically therewith. Particles of grinding sand are jetted onto the rotating rectifier element through a sand blast nozzle connected to a sand supply means. Jetted particles of sand grind and remove the semiconductor to form the ring-shaped groove 2.
This sand blast process has, however, the following drawbacks. Firstly, the radius R of curvature of groove bottom becomes large, as shown in FIG. 2, thus making the voltage durable characteristic worse. Secondly, processing accuracy is low, making uncertain the value of angle .theta. formed by the wall 2a located on the center side of the rectifier element and the PN junction J2. Therefore, the voltage durable characteristic is different for every rectifier element. Thirdly, processing efficiency is low. Fourthly, particles of sand are scattered during the process, thus making the working circumstance worse and providing a fear that the health of working persons can be injured.